With the advancement of new energy power, a large number of distributed power sources such as wind power and photovoltaic power are connected to the distribution network. Due to factors such as geography, weather, and season, the uncertainty and randomness of new energy power generation have brought new problems and challenges to the power grid. As an important component of a new type of power system, distributed energy storage systems have the characteristics of fast power throughput, high control accuracy, and flexible installation, making it easier to achieve rapid on-site consumption of distributed energy. In addition, compared to centralized energy storage systems, distributed energy storage reduces the line loss and investment pressure of electrical energy, and can achieve functions such as frequency regulation and peak shaving, delaying power grid transformation, and improving power grid operation stability.
In order to fully explore the value of distributed energy storage systems to the distribution network and improve the economy and reliability of power grid operation, it is necessary to deeply understand and analyze the grid connection characteristics and precautions of distributed energy storage systems, implement active management and control of distributed energy storage systems, and ensure their safe and stable operation. Therefore, relevant standards have put forward principled technical requirements for the design, operation, maintenance, testing, and scheduling of the access system for distributed power sources connected to the distribution network, in order to maximize the positive benefits of distributed power grid connection. Based on the possible problem of grid connected switchgear circuit breakers not closing due to voltage loss during the off grid operation of distributed energy storage systems, the action characteristics and detection mechanism of circuit breaker voltage loss are analyzed, and an improvement plan is proposed, which has high promotion and application value in engineering applications.
1.General technical principles for distributed power supply access
Chapter 11 of the State Grid Enterprise Standard Q/GDW 10370-2016 “Technical Guidelines for Distribution Networks” provides guidelines for the access of distributed power sources, including access capacity, grid connection voltage, access system scheme, grid connection interface, and device functional requirements.
In terms of the requirements for grid connection interfaces and device functions, the technical guidelines clearly state that the 380 (220) V user incoming metering device connected to the distributed power supply and the boundary switch at the 10 (35) kV user public connection point should have functions such as grid side voltage loss delay tripping, voltage blocking and closing on one or both sides of the user, and automatic closing when there is voltage delay on the grid side to ensure that the grid equipment Equipment and personal safety of maintenance (emergency repair) operators and other customers on the same network. Among them, the setting value for the voltage loss tripping of the 380 (220) V user’s incoming line metering device should be set to 20% UN for 10 seconds, and the setting value for detecting voltage should be set to greater than 85% UN.
The technical requirements for connecting distributed power sources of the inverter type to the 220/380 V distribution network also clearly stipulate that the grid connection point should be equipped with a low-voltage grid connection dedicated switch that is easy to operate, has obvious disconnection indication, and has the ability to disconnect fault current. It should have the functions of voltage loss tripping and voltage detection and closing.
Therefore, circuit breakers used for connecting distributed power sources to public connection points must have a voltage loss tripping function. For the implementation of the voltage loss tripping function, the most commonly used method is to configure a voltage loss release. For grid connected protection circuits using molded case circuit breakers and frame circuit breakers, a voltage loss release can be used to achieve this function.
2.Hidden dangers of distributed energy storage off grid control issues
There are two typical operating states of energy storage systems. When the energy storage system is connected to the grid, it is connected to the microgrid and operates in PQ mode; During off grid operation, the energy storage system provides voltage and frequency to support the operation of the microgrid, operating in VF mode. The typical operation topology diagram of the energy storage system shows that when running off the grid, the grid measurement circuit breaker Q1 is opened, and the grid connected cabinet circuit breaker Q2 of the energy storage system is closed, providing electrical energy support for the load.
In order to standardize the design of distributed energy storage system access system and ensure the safe and stable operation of energy storage system and distribution network, the China Electric Power Enterprise Federation T/CEC 0231-2017 “Design Specification for Distributed Energy Storage System Access to Distribution Network” Chapter 6 Secondary System Design specifies that the distributed energy storage system should have emergency disconnection function in case of abnormal frequency and voltage, which is achieved by a dedicated installation automatic device, It can also be achieved by a fast switch of the parallel network with this function.
At present, in distributed photovoltaic systems, the circuit breakers and other switches of the grid connected switchgear used for energy storage systems have a voltage loss tripping function. The most commonly used voltage loss tripping function is achieved by using a voltage loss release. The voltage loss tripping function detects the voltage on the port (grid side) of the circuit breaker. When the grid loses voltage and power is cut off, the circuit breaker trips. When the energy storage system is running off the grid, due to the power loss of the grid, there is no voltage at the upper port (grid side) of the grid connected switchgear circuit breaker, and only at the lower port (energy storage converter and battery end) of the circuit breaker, which will cause the circuit breaker of the grid connected switchgear to lose power and trip during the off grid operation, making it impossible to perform off grid operation.
Therefore, the distributed photovoltaic configuration energy storage system takes into account the special needs of energy storage off grid control, and may encounter the problem of the common connection point circuit breaker and other switches not effectively closing when off grid. Therefore, it is necessary to analyze the action characteristics and detection mechanism of the circuit breaker’s voltage loss release.
3.Action characteristics and detection mechanism of voltage loss release
3.1 Action characteristics of voltage loss release
For circuit breakers that achieve voltage loss tripping through a voltage loss release, their conventional voltage loss release action characteristics.
1) When the applied voltage suddenly drops to within the range of 35% to 10% of the rated working voltage, the voltage loss release releases and disconnects the circuit breaker.
2) When the applied voltage is lower than 35% of the rated working voltage of the voltage loss release, the voltage loss release prevents the circuit breaker from closing.
3) When the applied voltage is 85% to 110% of the rated working voltage of the voltage loss release, the voltage loss release can ensure the reliable closing of the circuit breaker.
4) When the applied voltage drops and is not less than 35% of the rated working voltage, the voltage loss release will be closed to ensure the reliable closing of the circuit breaker.
According to the operating characteristics of the circuit breaker and in combination with the relevant provisions of Q/GDW 10370-2016 “Technical Guidelines for Distribution Networks”, voltage loss trip detection ensures that in the event of a sudden drop in the rated working voltage of the system to 20%, the distributed power supply circuit breaker disconnects from the power grid system and enters a fault or off grid operation state.
3.2 Detection mechanism of voltage loss release
The undervoltage release mainly includes two parts: the electromagnetic system and the control circuit. Among them, the electromagnetic system is mainly composed of an iron core, a spring, a suction coil, etc. The detection circuit is mainly used to detect the amplitude of the phase voltage at the upper port (grid voltage terminal) of the circuit breaker. When the voltage suddenly drops to the operating voltage, the undervoltage release control circuit quickly detects the voltage drop and provides a tripping command. The electromagnetic actuator coil loses power, and the push rod impacts the release plate under the action of the spring to make it act, thus tripping the circuit breaker and providing protection.
4.Improvement plan for circuit breakers used in energy storage systems
In response to the issue of the grid connected switchgear circuit breaker tripping due to voltage loss release of the energy storage system under off grid conditions, and the circuit breaker being unable to close, this article adds a voltage loss trip detection point for the grid connected switchgear. At the same time as detecting voltage loss at the port (grid measurement) detection point on the original grid connected mode circuit breaker, a transfer switch is used to set the off grid mode. When the energy storage system is operating in off grid state, The circuit breaker voltage loss release detects the voltage at the lower port of the circuit breaker. At this point, the lower port of the grid connected switchgear circuit breaker of the energy storage system in an off grid state is energized, and the voltage loss trip detection point of the circuit breaker will no longer report a voltage loss trip fault. The circuit breaker has the function of normal operation in both grid connected and off grid modes.
When the energy storage system operates in grid connected mode, the parallel off grid mode conversion switch is converted to grid connected mode. At this time, the circuit breaker of the grid connected switchgear detects the voltage at the upper port (measured by the power grid) according to conventional detection methods, and the circuit breaker can effectively perform live closing and power loss tripping functions. When the energy storage system operates in off grid mode, the parallel off grid mode conversion switch is switched to off grid mode. At this time, the voltage at the lower port (PCS and battery side) of the grid connected cabinet circuit breaker is detected, and the energy storage system operates off grid. The energy storage battery and energy storage converter work, and the lower port of the grid connected switchgear circuit breaker is charged, which can effectively close the circuit breaker. The energy storage system can stably operate under load under island off grid conditions.
It should be emphasized in the schematic diagram of circuit breaker voltage loss release detection under grid separation conditions that the circuit breaker of the parallel cabinet used in the photovoltaic system should have the functions of voltage loss tripping and voltage detection closing, and the energy storage system should also have the function of voltage loss tripping. However, for safety reasons, the energy storage system cannot have the function of voltage detection closing to prevent personal and equipment damage caused by operations such as energy storage off grid operation. At the same time, distribution cabinet circuit breakers connected to public connection points based on hybrid microgrids such as optical storage should not have the function of detecting and closing voltage.
5.Application
A microgrid system in a certain park includes two subsystems. The first subsystem is a 712 kW photovoltaic power generation system, while the second subsystem is an intelligent microgrid composed of 285 kW photovoltaic, 250 kW/600 kWh energy storage, one 5 kW wind turbine, and two 160 kW DC charging piles. Among them, the circuit breaker for the grid connected cabinet of the energy storage system is Q1, and the circuit breaker for the grid connected cabinet of the wind and solar energy storage and charging microgrid is Q2.
When the power grid loses power, the microgrid energy storage system operates in an off grid state. As the energy storage system uses the voltage loss tripping release of circuit breaker Q1 to detect the voltage at the upper end of the circuit breaker, the upper end cannot be closed due to the voltage loss alarm of the circuit breaker caused by the power grid losing power. Therefore, the above circuit breaker modification scheme is adopted. While detecting the voltage loss at the upper port of the original circuit breaker, the off grid mode is set using a transfer switch. During the off grid control of the energy storage system, The circuit breaker voltage loss release detects the voltage at the lower port of the circuit breaker. At this point, the energy storage battery serves as the energy source, and the energy storage converter operates in the off grid stable voltage mode. The lower port of circuit breaker Q1 is charged, and the circuit breaker can effectively close. The microgrid system can operate off the grid.
The circuit breaker Q2 of the grid connected cabinet at the public connection point of the microgrid adopts the same improvement scheme by adding an off grid transfer switch and adding a voltage loss trip point detection at the lower port of the circuit breaker, ensuring that the entire microgrid system can reliably operate off the grid in the event of power loss in the large grid.
Starting from the possible problem of grid connected switchgear circuit breakers not being able to close due to voltage loss during the off grid operation of distributed energy storage systems, this paper analyzes the general technical principles of distributed power supply connection, elaborates on the action characteristics and detection mechanism of switchgear circuit breaker voltage loss release during the off grid operation of energy storage systems, and provides a plan to increase the detection points of grid connected conversion switches and voltage loss releases, Ensure that the grid connected cabinet circuit breaker of the energy storage system can operate reliably when it is disconnected from the grid.
The problem of grid connected cabinet circuit breaker off grid closing occurred in the off grid control project of a solar energy storage and charging microgrid in a certain park. Through the above methods and improvement measures, the project was reliably closed and effectively realized under off grid conditions. The feasibility of this method has been verified from engineering practice, and it has good promotion value.